Process for the simultaneous production of maleic anhydride and its hydrogenated derivatives

ABSTRACT

A process is described for the co-production of maleic anhydride and at least one C 4  compound selected from butane-1,4-diol, γ-butyrolactone, and tetrahydrofuran in which maleic anhydride is produced by partial oxidation of a hydrocarbon feedstock selected from C 4  hydrocarbons and benzene to yield a vaporous reaction effluent stream comprising maleic anhydride, water, unconverted hydrocarbon feedstock, and carbon oxides. A part of the maleic anhydride present in the vaporous reaction effluent stream is condensed to form a crude maleic anhydride stream and leave a residual vaporous stream containing residual amounts of maleic anhydride. Further maleic anhydride is absorbed from the residual vaporous stream by absorption in an organic solvent, water or an aqueous solution. Maleic anhydride is then recovered from the loaded liquid absorptions medium. Said at least one C 4  compound is produced by hydrogenation of a C 4+  hydrogenation feedstock selected from maleic anhydride, maleic acid, dialkyl maleates, and mixtures of two or more thereof. The process is characterised in that material of the crude maleic anhydride stream is used as the C 4+  hydrogenation feedstock or is used to prepare the C 4+  hydrogenation feedstock.

This application is a 371 of PCT/GB00/03805, filed Oct. 4, 2000.

This invention relates to a process for the co-production of C₄compounds, more specifically maleic anhydride, butane-1,4-diol,γ-butyrolactone, and tetrahydrofuran, from a hydrocarbon feedstockselected from C₄ hydrocarbons and benzene.

Maleic anhydride can be produced by vapour phase oxidation of ahydrocarbon feedstock, such as benzene, mixed C₄ olefins, or n-butane,in the presence of a partial oxidation catalyst.

Depending on the nature of the feedstock a supported promoted vanadiumpentoxide catalyst is typically used, while the reaction temperature isusually from about 350° C. to about 500° C. and the reaction pressure isfrom about 10⁵ Pa to about 3×10⁵ Pa. A substantial excess amount of airmay be used in order to stay outside the explosive limits. The contacttime is about 0.1 s. Alternatively it is possible, according to moremodern practice, to design the plant so that satisfactory safe operationcan be achieved, despite the fact that the feed mixture of air andhydrocarbon feedstock is within the flammable limits.

One design of reactor for such partial oxidation reactions comprises atubular reactor including vertical tubes surrounded by a jacket throughwhich a molten salt is circulated in order to control the reactiontemperature. However, other designs of reactor can be used instead,including fixed bed reactors, fluidised bed reactors, or moving bedreactors.

In each case a hot vaporous reaction mixture is recovered from the exitend of the reactor which comprises maleic anhydride vapour, watervapour, carbon oxides, oxygen, nitrogen, and other inert gases, besidesorganic impurities such as acetic acid, acrylic acid, and unconvertedhydrocarbon feedstock.

It is usual to recover and purify the maleic anhydride from this dilutereactor effluent stream in up to four steps. First, in an optional step,some conventional processes condense out part of the maleic anhydride bycooling the reactor effluent stream, typically to about 150° C. using asteam-producing heat exchanger and then cooling it further to about 60°C. by cooling it against water, in order to condense part of the maleicanhydride, typically about 30% to about 60% of the maleic anhydridepresent. Only partial condensation is effected because of the presenceof water which reacts with maleic anhydride in the reactor effluentstream to form maleic acid, which may in turn isomerise to fumaric acid.Maleic acid has a melting point of 130° C., while fumaric acid has amelting point of 287° C., both of which are much higher than that ofmaleic anhydride (52.85° C.). As a result there is a tendency fordeposits of solid maleic acid and fumaric acid to build up on the heatexchanger surfaces which require periodic removal, typically by use ofwater and/or sodium hydroxide solution which yields an aqueous solutionthat contains fumaric acid and maleic acid or their sodium salts andrequires effluent treatment.

A second step that is conventionally used is to absorb essentially allof the remaining maleic anhydride from the effluent stream. Theremaining gaseous effluent can then be vented to the atmosphere,possibly after incineration of carbon monoxide, unconverted hydrocarbon,and other organic compounds contained therein. In this absorption stepan organic solvent can be used. Alternatively an aqueous solution can beused as the absorbent, in which case the maleic anhydride is mainlyhydrolysed to form maleic acid.

Scrubbing with water or with an aqueous solution or slurry is described,for example, in U.S. Pat. No. 2,638,481. A disadvantage of such aprocedure, however, is that some of the maleic acid is inevitablyisomerised to fumaric acid. The byproduct fumaric acid represents a lossof valuable maleic anhydride and is difficult to recover from theprocess system since it tends to form crystalline masses which give riseto process fouling problems.

Because of this isomerisation problem a variety of other anhydrousorganic solvents have been proposed for absorption of maleic anhydridefrom vaporous streams, for example, dibutyl phthalate (British PatentSpecifications Nos. 727,828, 763,339, and 768,551), dibutyl phthalatecontaining up to 10 weight % phthalic anhydride (U.S. Pat. No.4,118,403) normally liquid intramolecular carboxylic acid anhydrides,such as a branched chain C₁₂₋₁₅-alkenyl substituted succinic anhydride(U.S. Pat. No. 3,818,680), tricresyl phosphate (French PatentSpecification No. 1,125,014), dimethyl terephthalate (Japanese PatentPublication No. 32-8408), dibutyl maleate (Japanese Patent PublicationNo. 35-7460), high molecular weight waxes (U.S. Pat. No. 3,040,059),diphenylpentachloride (U.S. Pat. No. 2,893,924), high boiling aromatichydrocarbon solvents, such as dibenzylbenzene (French PatentSpecification No. 2,285,386), dimethylbenzophenone (U.S. Pat. No.3,850,758), polymethylbenzophenones, at least a portion of which containat least 3 methyl groups, (U.S. Pat. No. 4,071,540), water-insolubletertiary amines (U.S. Pat. No. 4,571,426), dialkyl phthalate estershaving C₄ to C₈ alkyl groups and a total of 10 to 14 carbon atoms inboth alkyl groups (U.S. Pat. No. 3,891,680), and esters ofcycloaliphatic acids, for example dibutyl hexahydrophthalate (SouthAfrican Patent Specification No. 80/1247).

A third step that is conventionally used is to recover the resultingsolution of maleic anhydride or maleic acid from the absorbent. When theabsorbent is an organic solvent, batch distillation or continuousdistillation can be used to recover the maleic anhydride. On the otherhand, when the absorbent liquid is water or an aqueous solution, therecovery step must include a dehydration step so as to re-convert themaleic acid back to maleic anhydride. One procedure that is used is todistil the maleic acid solution in the presence of xylene. This not onlyremoves the water but also results in re-formation of maleic anhydride.In either event the elevated temperatures used tend to induce formationof fumaric acid which constitutes a further loss of potential productmaleic anhydride.

U.S. Pat. No. 5,069,687 proposes recovery of maleic anhydride from agaseous mixture by contact with an absorbent, following which water isremoved from the enriched absorbent by contacting it with a wateradsorbent or with a low humidity stripping gas. Maleic anhydride is thenrecovered from the dried enriched absorbent.

A growing use for maleic anhydride is in the production ofbutane-1,4-diol, and its co-products, i.e. γ-butyrolactone, andtetrahydrofuran. Direct hydrogenation of maleic anhydride or maleic acidto these C₄ compounds is proposed in U.S. Pat. Nos. 3,948,805,4,001,282, 4,048,196, 4,083,809, 4,096,156, 4,550,185, 4,609,636,4,659,686, 4,777,303, 4,985,572, 5,149,680, 5,347,021, 5,473,086, and5,698,749, and in European Patent Publication No. 0373947A.

Esterification of maleic anhydride with an alkyl alcohol to yield adialkyl maleate followed by hydrogenation of the resulting dialkylmaleate has also been proposed in order to produce butane-1,4-diol, andits co-products, γ-butyrolactone and tetrahydrofuran. Hydrogenation inthe liquid phase is proposed in British Patent Specification No.1,454,440. Vapour phase hydrogenation is taught in International PatentPublication No. WO 82/03854. Hydrogenation of a dialkyl maleate in twostages can be carried out as described in U.S. Pat. Nos. 4,584,419 and4,751,334.

U.S. Pat. No. 4,032,458 proposes esterification of maleic acid with a C₂to C₁₀ alkanol at elevated pressure and temperature followed by a twostage hydrogenation of the resulting dialkyl maleate using a slurry of acopper chromite catalyst and then by distillation.

U.S. Pat. No. 5,478,952 suggests a hydrogenation catalyst which can beused in aqueous solution to hydrogenate, for example, maleic acid, andwhich consists of a mixture of ruthenium and rhenium on carbon.

Processes and plant for the production of dialkyl maleates from maleicanhydride are described, for example, in U.S. Pat. No. 4,795,824 and inInternational Patent Publication No. WO 90/08127. This last mentioneddocument describes a column reactor containing a plurality ofesterification trays each having a predetermined liquid hold-up andcontaining a charge of a solid esterification catalyst, such as an ionexchange resin containing pendant sulphonic acid groups.

The hydrogenation of dialkyl maleates to yield butane-1,4-diol isdiscussed further in U.S Pat. Nos. 4,584,419, and 4,751,334, andInternational Patent Publication No. WO 88/00937.

In International Patent Publication No. WO 97/43242 a process isdescribed in which maleic anhydride is absorbed in a high boilingsolvent having a boiling point that is at least 30° C. higher than thatof maleic anhydride at atmospheric pressure, for example dimethylphthalate. Then the maleic anhydride in the resulting solution isesterified to form the corresponding di-(C₁ to C₄ alkyl) maleate, whichis subsequently stripped from the solution using a hydrogen-containinggas stream to yield a vaporous mixture which is then subjected to vapourphase hydrogenation. A similar procedure in which the esterificationstep is omitted and the maleic anhydride is stripped from the solutionin the high boiling solvent and subjected to vapour phase hydrogenationis described in International Patent Publication No. WO 97/43234.Further materials for use as absorption solvent are taught inInternational Patent Publications Nos. WO 99/25675 and WO 99/25678.

A further development of such processes is proposed in InternationalPatent Publication No. WO 99/48852; in this development a second highboiling solvent, such as dibutyl phthalate, is used to scrub the off-gasfrom an absorption step in which maleic anhydride is absorbed from acrude vaporous maleic anhydride stream from a maleic anhydride plant ina first high boiling solvent, such as dimethyl phthalate.

In the prior art processes for production of butane-1,4-diol from maleicanhydride it is normal procedure to utilise a substantially pure maleicanhydride feedstock which contains at most a trace each of light acids(e.g. acetic acid and acrylic acid), of fumaric acid and of maleic acid.

It is an object of the present invention to provide an improved processfor the co-production of maleic anhydride and the C₄ compounds,butane-1,4-diol, γ-butyrolactone, and tetrahydrofuran. It is also anobject of the present invention to improve the yield of such C₄compounds from a given quantity of hydrocarbon feedstock and hence tomake these compounds more readily available and to reduce the quantityof waste products produced. It is also an object of the presentinvention to provide a process for the production of the C₄ compounds,maleic anhydride, butane-1,4-diol, γ-butyrolactone, and tetrahydrofuranfrom a hydrocarbon feedstock which can be operated in a plant that ismore economical to construct and to run than conventional plants.

According to the present invention there is provided a process for theco-production of maleic anhydride and at least one C₄ compound selectedfrom butane-1,4-diol, γ-butyrolactone, and tetrahydrofuran wherein:

maleic anhydride is produced by steps comprising:

(i) supplying a source of gaseous oxygen and a hydrocarbon feedstockselected from C₄ hydrocarbons and benzene to a catalytic partialoxidation zone which contains a charge of a partial oxidation catalystcapable of effecting the partial oxidation of the hydrocarbon feedstockto form maleic anhydride and which is maintained under catalytic partialoxidation conditions;

(ii) recovering from the partial oxidation zone a vaporous reactioneffluent stream comprising maleic anhydride, water, unconvertedhydrocarbon feedstock, and carbon oxides;

(iii) condensing a part of the maleic anhydride present in the vaporousreaction effluent stream in a condensation zone to form a crude maleicanhydride stream;

(iv) recovering from the condensation step (iii) a residual vaporousstream containing residual amounts of maleic anhydride;

(v) absorbing further maleic anhydride from the residual vaporous streamof step (iv) by absorption in a liquid absorption medium selected froman organic solvent, water, and an aqueous solution;

(vi) recovering from the absorption step (v) a loaded liquid absorptionmedium; and

(vii) recovering maleic anhydride from the loaded liquid absorptionmedium; and wherein:

said at least one C₄ compound selected from butane-1,4-diol,γ-butyrolactone, and tetrahydrofuran is produced by steps comprising:

(viii) providing a C₄₊ hydrogenation feedstock selected from maleicanhydride, maleic acid, dialkyl maleates, and mixtures of two or morethereof;

(ix) supplying said C₄₊ hydrogenation feedstock and hydrogen to ahydrogenation zone which contains a charge of a hydrogenation catalysteffective for catalytic hydrogenation of the C₄₊ hydrogenation feedstockto yield said at least one C₄ product and which is maintained undercatalytic hydrogenation conditions; and

(x) recovering from the hydrogenation zone a hydrogenation productstream containing said at least one C₄ product;

characterised in that crude maleic anhydride condensate of the crudemaleic anhydride stream of step (iii) is used as the C₄₊ hydrogenationfeedstock of step (viii) or is used to prepare the C₄₊ hydrogenationfeedstock of step (viii).

The source of gaseous oxygen may comprise substantial amounts of inertgases, such as nitrogen, in addition to oxygen. Air is a convenientsource of gaseous oxygen for use in the process of the invention. Hencethe vaporous reaction effluent stream of step (ii) may contain nitrogenand oxygen in addition to the other components mentioned. It will oftenbe expedient to cool the vaporous reaction effluent stream of step (ii)prior to attempting to effect condensation in step (iii).

In a preferred process according to the invention all of the crudemaleic anhydride reaction product of step (iii) is used as the C₄₊hydrogenation feedstock of step (viii) or is used to prepare the C₄₊hydrogenation feedstock of step (viii).

If necessary, at least some of the maleic anhydride recovered in step(vii) can be used as or to prepare further C₄₊ hydrogenation feedstockfor use in step (viii).

Alternatively, if the liquid absorption medium used in step (v) is wateror an aqueous solution so that the loaded absorption solution containsmaleic acid, at least some of the maleic acid present in the loadedabsorption medium can be used as or to prepare further C₄₊ hydrogenationfeedstock for use in step (viii). In this case at least some of saidresulting maleic acid can first be concentrated by removal of excesswater prior to use in step (viii). Moreover maleic acid present in theloaded absorption medium, whether excess water is removed forconcentration purposes or not, can be mixed with crude maleic anhydrideof step (iii) for use as or to prepare the C₄₊ hydrogenation feedstockof step (viii).

In a preferred process according to the invention, condensation ofmaleic anhydride is effected in step (iii) by indirect cooling against acooling medium selected from water and a process fluid. In analternative preferred process condensation of maleic anhydride iseffected in step (iii) in the presence of a liquid condensation mediumcomprising a liquid selected from maleic anhydride, monoesters of maleicacid, diesters of maleic acid, and mixtures of two or more thereof, inorder to reduce the fouling of the condenser surfaces. Thus directcooling may be carried out by spraying the liquid condensation mediuminto the vaporous reactant effluent stream so as to form a mixture ofcrude maleic anhydride and said liquid condensation medium which is usedas or to prepare the C₄₊ hydrogenation feedstock of step (viii). Suchmonoesters and diesters can be derived, for example, from C₁ to C₄ alkylalcohols such as methanol and ethanol. Moreover the liquid condensationmedium can also contain small amounts, e.g. up to about 5 molar %, ofthe corresponding monoalkyl and dialkyl fumarates.

In one particularly preferred process the C₄₊ hydrogenation feedstock isa dialkyl maleate which is prepared by reaction of maleic anhydride withan alkyl alcohol to form a monoalkyl maleate which is then esterifiedwith further alkyl alcohol to form a dialkyl maleate. In this case thealkyl alcohol can be methanol or ethanol and the dialkyl maleate can bedimethyl maleate or diethyl maleate. In this case the hydrogenationcatalyst of step (ix) is preferably selected from copper chromite andpromoted copper catalysts, such as manganese promoted copper catalysts.

In an alternative preferred process according to the invention the C₄₊hydrogenation feedstock is maleic anhydride. In this case the catalystcan be any one of those proposed for the purpose in the prior art, forexample one of the catalysts disclosed in one of the aforementioned U.S.Pat. Nos. 3,948,805, 4,001,282, 4,048,196, 4,083,809, 4,096,156,4,550,185, 4,609,636, 4,659,686, 4,777,303, 4,985,572, 5,149,680,5,473,086, 5,478,952, and 5,698,749.

Conveniently the source of gaseous oxygen is air. However, mixtures ofnitrogen and air, mixtures of off gas and air, mixtures of off gas andoxygen, pure oxygen, and oxygen-enriched air may also be mentioned asthe source of oxygen. The off gas may comprise that part of the residualvaporous gas that remains after absorption of further maleic anhydridein the liquid absorption medium in step (v). The source of gaseousoxygen may be used in excess so as to maintain the mixture ofhydrocarbon feedstock and source of gaseous oxygen, e.g. air, outsideflammable limits. Alternatively the process may be operated so that themixture is within flammable limits.

In one particularly preferred process the hydrocarbon feedstock is abutane feedstock.

In this case the partial oxidation catalyst may comprise avanadium-phosphorus-oxide catalyst. Such a catalyst is sometimesdescribed as vanadyl pyrophosphate. In order to maintain the activity ofthe catalyst volatile organophosphorus compounds can be bled into thefeed mixture to the catalytic partial oxidation zone. As some phosphoruscompounds may be present in the vaporous reaction effluent stream ofstep (ii) of the process of the invention which may deactivate thehydrogenation catalyst of step (ix), a guard bed of aphosphorus-absorbing material, such as vanadium-containing material;conveniently a charge of spent partial oxidation catalyst, may in thiscase be placed in the path of the vaporous reaction stream of step (ii),after cooling thereof, in order to remove phosphorus-containingmaterials therefrom.

In an alternative process the feedstock comprises benzene. In this casethe catalyst can be, for example, a supported vanadium pentoxidecatalyst which may be modified with molybdenum oxide.

The catalytic partial oxidation zone may be of any suitable design, forexample it may be a fixed bed reactor, a tubular reactor, a fluidisedbed reactor, or a moving bed reactor.

Step (v) of the process of the invention may comprise absorbing vaporousmaleic anhydride from the effluent stream in an organic solvent, such asa dialkyl phthalate or a dialkyl hexahydrophthalate, for exampledimethyl phthalate, dibutyl phthalate, dimethyl hexahydrophthalate, ordibutyl hexahydrophthalate. Water or an aqueous solution of maleic acidcan alternatively be used to absorb vaporous maleic anhydride from theeffluent stream in step (v).

In the process of the invention some of the maleic anhydride recoveredin step (vii) may be used as, or may be used to make, additional C₄₊hydrogenation feedstock of step (viii).

It will often be preferred, during shutdown, to use an alkyl alcohol aswash liquor to wash condensation surfaces of the condensation zone toremove deposits of fumaric acid thereon and to combine the resultingsolution with the C₄₊ hydrogenation feedstock. This procedure has theadvantage of avoiding or reducing the use of sodium hydroxide and theproduction of aqueous byproduct streams containing maleic acid or sodiummaleate which are a feature of washing procedures using water and/orsodium hydroxide solution to remove fouling deposits of maleic acid andfumaric acid from condenser surfaces.

In order that the invention may be clearly understood and readilycarried into effect a conventional plant for co-production of maleicanhydride and of the C₄ products, butane-1,4-diol, γ-butyrolactone, andtetrahydrofuran and a plant for the same purpose built according to theteachings of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, wherein:

FIG. 1 is a flow diagram of a conventionally designed plant for theco-production of maleic anhydride and of the C₄ products,butane-1,4-diol, γ-butyrolactone, and tetrahydrofuran; and

FIG. 2 is a similar flow diagram of a plant constructed in accordancewith the teachings of the invention and intended for the same purpose.

It will be appreciated by those skilled in the art that, since theaccompanying drawings are diagrammatic, many other items of equipmentwhich are not shown in the drawings would be required in an actualplant. Such additional items of equipment are conventional in nature andinclude (but are not limited to) distillation columns, reactors,condensers, pumps, holding tanks, valves, pressure sensors, temperaturesensors, pressure controllers, temperature controllers, level sensors,heaters, coolers, surge tanks, condensers, column reboilers, and thelike. Any such additional items of equipment would be installed inaccordance with conventional engineering practice and form no part ofthe present invention.

Referring to FIG. 1 of the drawings, a partial oxidation unit 1comprises a partial oxidation reactor containing a charge of a partialoxidation catalyst, for example a vanadium-containing catalyst such asvanadium-phosphorus-oxide (whose catalytically active phase has beenreported to be vanadyl pyrophosphate, (VO)₂P₂O₇). Unit 1 can be of fixedbed or fluidised bed design and is supplied with a superheated C₄hydrocarbon feedstock, such as butane, by means of line 2 and with airby means of line 3. The butane:air volume ratio is typically from about20:1. The catalyst charge in partial oxidation unit 1 is maintainedunder a pressure of from about 100 kPa to about 200 kPa.

In partial oxidation unit 1 butane is converted by partial oxidation tomaleic anhydride. The main byproducts are carbon monoxide, carbondioxide, and water:

All three reactions are highly exothermic.

For further details of suitable designs of partial oxidation unit forproduction of maleic anhydride reference may be made to Kirk-OthmerEncyclopedia of Chemical Technology, Fourth Edition, Volume 15, pages893 to 928.

A hot vaporous partial oxidation reaction product stream is recoveredfrom partial oxidation unit in line 4. This is at a temperature of fromabout 390° C. to about 430° C. and contains nitrogen, oxygen, unreactedbutane, water, carbon oxides, maleic anhydride, maleic acid, fumaricacid, and traces of acetic acid, and acrylic acid. The reaction productstream is then cooled before it is fed to a condensation stage 5 whichis maintained at a temperature below the dew point of the partialoxidation product stream so as to cause from about 20% to about 60%,preferably from about 40% to about 60%, of the maleic anhydride presentin line 4 to condense. Typically condensation stage 5 includes twocoolers, the first operating at about 150° C. and the second at about60° C. The resulting condensate is recovered in line 6.

The residual maleic anhydride, which is still in vaporous form, passeson in line 7 to an absorption unit 8, for example a gas scrubber unit,through which the vapour stream is passed upward in countercurrent to adown flowing stream of an organic solvent, such as dibutyl phthalate,supplied from line 9. The residual gas exits absorption unit 8 in line10 and can be vented, typically via an off gas incinerator, while theresulting solution of maleic anhydride is recovered in line 11 andpassed to a solvent recovery unit 12. In recovery unit 12 the solvent isseparated from the maleic anhydride, conveniently by distillation undernormal or reduced pressure. The recovered solvent is recycled in line 9while the separated maleic anhydride is passed on by way of line 13 to apurification unit 14 for further purification, for example by batchdistillation. Some of the resulting product maleic anhydride isrecovered in line 15 while the remainder is passed by means of line 16to a butane-1,4-diol plant 17.

Reference numerals 18 and 19 represent lines for the recovery of lightimpurities and heavy impurities respectively.

In the butane-1,4-diol plant 17 maleic anhydride is reacted with analkyl alcohol, such as methanol, to form a monoalkyl maleate, such asmonomethyl maleate, which is then reacted further in a countercurrentreactor column of the type described in European Patent Publication No.0454719A for substantially complete conversion to dimethyl maleate.

The reactions concerned are:

The monoesterification reaction (1) is autocatalytic but reaction (2) ispreferably conducted in the presence of an acidic esterificationcatalyst such as an ion exchange resin containing sulphonic acid groups.For further details the attention of the reader is directed to EuropeanPatent Publication No. 0454719A. Other specifications describingproduction of dialkyl maleates are European Patent Specification Nos.0255399A and 0255401A. The resulting dimethyl maleate is thenhydrogenated in the vapour phase using a copper chromite catalyst or apromoted copper catalyst, such as a manganese promoted catalyst of thetype disclosed in European Patent Publication No. 0656336A, to yieldbutane-1,4-diol and, as co-products, γ-butyrolactone andtetrahydrofuran.

In addition the hydrogenation product mixture will normally containminor amounts of the corresponding dialkyl succinate, n-butanol, water,and a cyclic acetal, i.e. 2-(4′-hydroxybutoxy)-tetrahydrofuran of theformula:

The mechanisms for formation of all these products and by-products havenot been fully elucidated. However, their production is consistent withthe following reaction scheme:

wherein R is methyl. The methanol released in the course of thehydrogenation step is condensed and separated from the other condensablecomponents, including butane-1,4-diol, γ-butyrolactone, tetrahydrofuran,water and by-products including n-butanol. For further informationregarding methods of separating the condensable components of thehydrogenation reactions, reference may be made, for example, toInternational Patent Publications Nos. WO 91/01981 and WO 97/36846. Therecovered methanol can be recycled for production of further dimethylmaleate. Typical hydrogenation conditions include use of an H₂:dimethylmaleate molar ratio of from about 100:1 to about 400:1, for exampleabout 320:1, a temperature of from about 150° C. to about 240° C., and apressure of from about 5 bar (5×10⁵ Pa) to about 100 bar (10⁷ Pa),depending upon the desired butane-1,4-diol: γ-butyrolactone productratio.

Reference numeral 20 indicates a supply line for supplying methanol tothe butane-1,4-diol plant 17, while line 21 is the hydrogen supply linefor supply of the hydrogen needed for hydrogenation of dimethyl maleate.A purge gas stream is taken in line 22 while water and otherby-products, including n-butanol and heavy byproducts are recovered byway of line 23. Reference numerals 24, 25, and 26 indicate lines for therecovery of butane-1,4-diol, γ-butyrolactone, and tetrahydrofuran,respectively.

FIG. 2 illustrates a plant constructed in accordance with the teachingsof the invention. In FIG. 2 like reference numerals are used to indicatelike parts to those present in the plant of FIG. 1.

In the plant of FIG. 2 the process for the production of maleicanhydride is essentially the same as that of FIG. 1. However, instead ofsupplying substantially pure maleic anhydride in line 16 to thebutane-1,4-diol plant 17, this is instead supplied with crude maleicanhydride condensate from condensation unit 5 by way of line 27. Inaddition it can also be supplied, as and when necessary, with partiallyrefined or pure maleic anhydride in line 28 from solvent recovery unit12. Hence when the demand for butane-1,4-diol, γ-butyrolactone and/ortetrahydrofuran rises beyond the output determined by the flow rate ofmaleic anhydride in line 27, some of the maleic anhydride from solventrecovery unit 12 can be diverted to the butane-1,4-diol plant 17 to makeup the shortfall.

Instead of using an organic solvent, such as dibutyl phthalate, inabsorption unit 8 an aqueous absorbent, such as water or an aqueoussolution of maleic acid, can alternatively be used. In this case thestream in line 11 will be an aqueous solution of maleic acid, whilerecovery unit 12 is a dehydration unit in which the aqueous solution ofmaleic acid is subjected to dehydration either by heating or bydistillation in the presence of xylene to regenerate maleic anhydride,the water released by dehydration of maleic acid as well as the water ofthe absorbent being recovered as an overhead product and recycled backin line 9 to absorption unit 8.

It is alternatively possible to effect condensation of part of themaleic anhydride present in the hot partial oxidation product streamfrom partial oxidation unit 4 in the presence of a cooled stream ofmaleic anhydride or other C₄₊ hydrogenatable material, such as amonoalkyl maleate (e.g. monomethyl maleate), dialkyl maleate (e.g.dimethyl maleate), or a mixture of two or more thereof. At least a partof this stream can then be passed to the butane-1,4-diol plant 17. Theremainder can be cooled and used again for condensation. Alternativelythe entire stream can be passed forward to the butane-1,4-diol unit 17for conversion of the maleic anhydride present therein to dimethylmaleate, some of which is then recycled for condensation of furthermaleic anhydride. An advantage of such a procedure is that fouling ofsurfaces in the condensation stage may be reduced.

It is further possible to feed some of the solution from absorption unit8, by way of line 29 to the butane-1,4-diol plant. Any impuritiespresent in the crude maleic acid or maleic anhydride of the streams inline 27 or line 29 are separated out in the course of the processingsteps used in butane-1,4-diol plant 17. Recovered solvent is returned inline 30 to line 9.

The condensed crude maleic anhydride stream present in line 27 of theplant of FIG. 2 contains as impurities which are not normally present inthe purified maleic anhydride feedstock in line 16 of the plant of FIG.1. Such impurities comprise light boiling impurities (such as aceticacid and acrylic acid), heavy impurities, and maleic anhydridederivatives (including maleic acid and fumaric acid). When the crudemaleic anhydride is esterified prior to hydrogenation, the acidic lightboiling impurities will also be esterified, as well as the maleicanhydride derivatives. The resulting light boiling esters, such asmethyl acetate or ethyl acetate and methyl acrylate or ethyl acrylate,can be stripped from the resulting dialkyl maleate (e.g. dimethylmaleate or diethyl maleate) overhead, along with the water ofesterification when using, for example, the procedure described inEuropean Patent Publication No. 0454719A. Esterification of the maleicanhydride derivatives will produce a corresponding additional amount ofthe corresponding dialkyl maleate. Heavy boiling impurities will passthrough the esterification reactor along with the dialkyl maleate andwill be fed to the hydrogenation vaporiser, when vapour phasehydrogenation is used. The hydrogenation vaporiser can be operated so asto vaporise the dialkyl maleate only partially and to recycle theunvaporised material back to the vaporiser, thereby concentrating theheavy impurities in the unvaporised material and enabling the heavyimpurities to be removed in a purge stream.

In the case when the C₄₊ hydrogenation feedstock is the crude maleicanhydride stream, then the light impurities are also hydrogenated in thehydrogenation step and result in light hydrogenation products includingethanol and propanol, which can be removed from the hydrogenationproduct as an overhead product by distillation. Heavy impurities andheavy impurity hydrogenation products can be separated by conventionaldistillation techniques as a bottom product from the crude hydrogenationproduct. Maleic anhydride derivatives, such as maleic acid and fumaricacid, are hydrogenated to form a corresponding additional amount ofbutane-1,4-diol, γ-butyrolactone, and tetrahydrofuran.

The novel process of the invention has the important benefit that thecapital cost of the plant can be significantly reduced because it isnecessary to recover only about a half (typically about 40% to about60%) of the maleic anhydride in line 4. Hence the subsequent solventrecovery unit 12 and the purification section 14 can be correspondinglyreduced in size and can be operated with reduced operating costs.

The process of the invention also results in a higher efficiency ofmaleic anhydride usage because less maleic anhydride is lost as fumaricacid. Instead such fumaric acid is largely contained in the crudecondensate in line 27 and is converted to butane-1,4-diol,γ-butyrolactone and/or tetrahydrofuran in butane-1,4-diol plant 17.

Vanadium-phosphorus-oxide based catalysts are unstable in that they tendto lose phosphorus over time at reaction temperatures, this loss ofphosphorus having a tendency to accelerate if hot spots should developin a fixed bed reactor. Accordingly it may often be expedient to add avolatile organophosphorus compound to the partial oxidation catalystwith a view to providing catalyst activity stabilisation. In the courseof time volatile phosphorus compounds escape from the catalyst andappear in the vaporous reaction effluent stream from the partialoxidation unit. Since phosphorus is a potential catalyst poison orinhibitor for the hydrogenation catalyst used in the butane-1,4-diolunit 17 of the plant of FIG. 2, it is preferable in this case to includea guard bed of a phosphorus-absorbing material in line 4 to absorb anytraces of phosphorus-containing material that would otherwise have adeleterious effect upon the hydrogenation catalyst of butane-1,4-diolunit 17. Conveniently such a guard bed can be a bed of spentvanadium-containing partial oxidation catalyst. The temperature in sucha guard bed should be as low as possible in order to maximise phosphorusabsorption but above the condensation point for maleic anhydride in thestream of line 4.

The condensation surfaces of condensation unit 5 may become fouled withdeposits of maleic acid and fumaric acid. It is preferred to wash thesewith methanol. The resulting methanolic solution containing fumaricacid, maleic acid, and maleic anhydride, besides monomethyl maleate anddimethyl maleate formed by esterification, can be supplied as part ofthe feed to butane-1,4-diol unit 17. This has the advantage, comparedwith the potentially hazardous conventional water washing procedureusing water and/or sodium hydroxide solution that production of anaqueous solution of maleic acid and fumaric acid or sodium maleate andsodium fumarate is avoided.

In an alternative form of the process of the invention the C₄₊hydrogenation feedstock is maleic anhydride or maleic acid instead of adialkyl maleate, such as dimethyl maleate. In this case butane-1,4-diolplant 17 comprises a hydrogenation unit containing a hydrogenationcatalyst, for example one of the catalysts disclosed in the prior art asrepresented, for example, by the aforementioned U.S. Pat. Nos.3,948,805, 4,001,282, 4,048,196, 4,083,809, 4,096,156, 4,550,185,4,609,636, 4,659,686, 4,777,303, 4,985,572, 5,149,680, 5,473,086,5,478,952, and 5,698,749. Such catalysts are preferably used under theappropriate reaction conditions as disclosed in those prior artspecifications. The disclosures of all specifications mentioned aboveare herein incorporated by reference.

What is claimed is:
 1. A process for the co-production of maleicanhydride and at least one C₄ compound selected from the groupconsisting of butane-1,4-diol, γ-butyrolactone and tetrahydrofuranwherein: maleic anhydride is produced by steps comprising: supplying asource of gaseous oxygen and a hydrocarbon feedstock selected from C₄hydrocarbons and benzene to a catalytic partial oxidation zone whichcontains a charge of a partial oxidation catalyst capable of effectingthe partial oxidation of the hydrocarbon feedstock to form maleicanhydride and which is maintained under catalytic partial oxidationconditions; (ii) recovering from the partial oxidation zone a vaporousreaction effluent stream comprising maleic anhydride, water, unconvertedhydrocarbon feedstock, and carbon oxides; (iii) condensing a part of themaleic anhydride present in the vaporous reaction effluent stream in acondensation zone to form a crude maleic anhydride stream; (iv)recovering from the condensation step (iii) a residual vaporous streamcontaining residual amounts of maleic anhydride; (v) absorbing furthermaleic anhydride from the residual vaporous stream of step (iv) byabsorption in a liquid absorption medium selected from an organicsolvent, water, and an aqueous solution; (vi) recovering from theabsorption step (v) a loaded liquid absorption medium; and (vii)recovering maleic anhydride from the loaded liquid absorption medium;and wherein: said at least one C₄ compound selected from the groupconsisting of butane-1,4-diol, γ-butyrolactone and tetrahydrofuran isproduced by steps comprising: (viii) providing a C₄₊ hydrogenationfeedstock selected from maleic anhydride, maleic acid, dialkyl maleates,and mixtures of two or more thereof; (ix) supplying said C₄₊hydrogenation feedstock and hydrogen to a hydrogenation zone whichcontains a charge of a hydrogenation catalyst effective for catalytichydrogenation of the C₄₊ hydrogenation feedstock to yield said at leastone C₄ product and which is maintained under catalytic hydrogenationconditions; and (x) recovering from the hydrogenation zone ahydrogenation product stream containing said at least one C₄ product;wherein crude maleic anhydride condensate of the crude maleic anhydridestream of step (iii) is used as the C₄₊ hydrogenation feedstock of step(viii) or is used to prepare the C₄₊ hydrogenation feedstock of step(viii).
 2. A process according to claim 1, wherein all of the crudemaleic anhydride stream of step (iii) is used as the C₄₊ hydrogenationfeedstock of step (viii) or is used to prepare the C₄₊ hydrogenationfeedstock of step (viii).
 3. A process according to claim 1, wherein atleast some of the maleic anhydride recovered in step (vii) is used as orto prepare further C₄₊ hydrogenation feedstock for use in step (viii).4. A process according to claim 1, wherein the liquid absorption mediumused in step (v) is selected from water and an aqueous solution so thatthe loaded absorption solution contains maleic acid and that at leastsome of the maleic acid present in the loaded absorption medium is usedas or to prepare further C₄₊ hydrogenation feedstock for use in step(viii).
 5. A process according to claim 4, wherein at least some of saidresulting maleic acid is first concentrated by removal of excess waterprior to use in step (viii).
 6. A process according to claim 4, whereinmaleic acid present in the loaded absorption medium is mixed with crudemaleic anhydride of step (iii) for use as or to prepare the C₄₊hydrogenation feedstock of step (viii).
 7. A process according to claim1, wherein step (v) comprises absorbing vaporous maleic anhydride fromthe effluent stream in an organic solvent.
 8. A process according toclaim 7, wherein the organic solvent is a dialkyl phthalate or a dialkylhexahydrophthalate.
 9. A process according to claim 1, wherein in step(iii) condensation of maleic anhydride is effected by indirect coolingagainst a cooling medium selected from water and a process fluid.
 10. Aprocess according to claim 1, wherein in step (iii) condensation ofmaleic anhydride is effected in the presence of a liquid condensationmedium selected from maleic anhydride, monoesters of maleic acid, anddiesters of maleic acid, said liquid condensation medium being sprayedinto the vaporous reactant effluent stream so as to form a mixture ofcrude maleic anhydride and said liquid condensation medium which is usedas or to prepare the C₄₊ hydrogenation feedstock of step (viii).
 11. Aprocess according to claim 1, wherein the C₄₊ hydrogenation feedstock isa dialkyl maleate which is prepared by reaction of maleic anhydride withan alkyl alcohol to form a monoalkyl maleate which is then esterifiedwith further alkyl alcohol to form a dialkyl maleate.
 12. A processaccording to claim 11, wherein the alkyl alcohol is methanol and thedialkyl maleate is dimethyl maleate.
 13. A process according to claim11, wherein the hydrogenation catalyst of step (ix) is selected fromcopper chromite and promoted copper catalysts.
 14. A process accordingto claim 1, wherein the C₄₊ hydrogenation feedstock is maleic anhydride.15. A process according to claim 1, wherein the source of gaseous oxygenis air or a mixture of oxygen and recycle off gas.
 16. A processaccording to claim 1, wherein the hydrocarbon feedstock is a butanefeedstock.
 17. A process according to claim 16, wherein the partialoxidation catalyst comprises vanadium-phosphorus oxide.
 18. A processaccording to claim 17, wherein the vaporous reaction effluent stream iscooled prior to step (iii) and that a guard bed of aphosphorus-absorbing material is placed in the path of the cooledvaporous reaction effluent stream prior to step (iii) thereby to removephosphorus-containing materials therefrom.
 19. A process according toclaim 18, wherein the phosphorus-absorbing material comprises a chargeof spent vanadium-containing partial oxidation catalyst.
 20. A processaccording to claim 1, wherein the catalytic partial oxidation zonecomprises a fixed bed reactor, a tubular reactor, a fluidised bedreactor, or a moving bed reactor.
 21. A process according to claim 1,wherein an alkyl alcohol is used as wash liquor to wash condensationsurfaces of the condensation zone to remove deposits of maleic acid andfumaric acid thereon and that the resulting solution is combined withthe C₄₊ hydrogenation feedstock.